Zero Point SDX Problem: One Repeat Delay (FIXED!!)

[rude]

There is something wrong with the feedback on my Zero Point SDX. The ghost switch, the slam switch, and the feedback pot do not work.

I have checked my wiring, parts, and soldering numerous times. I have measured the voltages of the pins of IC1 (TL072) and IC2, IC3 (PT2399) with nothing unusual. I traced the signal with an audio probe and found that there was only one repeat in pin 7 (output) of IC1. I replaced the TL072 and still have only one repeat in pin 7 (output) of IC1.

[jimilee]

Welcome aboard! You got any pictures of both sides of the pcb and things of that nature?

[kothoma]

I traced the signal with an audio probe and found that there was only one repeat in pin 7 (output) of IC1.

OK then, last time someone had this problem it was a defective D4/D5, but it could also be a solder bridge.

Much of this discussion http://www.madbeanpedals.com/forum/index.php?topic=11805.msg106958#msg106958 should apply here too.

[muddyfox]

Awww man, with all the ZPSDX issues lately, I'm sure I'll run into a few more when I start my build. That board has been sitting on my desk for way too long.

[midwayfair]

What happens when you flip the path switch?

Do you have audio signal on pins 12-16 of BOTH PT2399s?

Audio probe the appropriate pins of the TL074 (filter section). Go through the modes one at a time. Verify that you have signal.

Also, use your multimeter's continuity settion to check all the pins on the rotary connection to verify that there aren't any solder bridges. I had that happen. You have to be very thorough and make sure nothing's connected when it shouldn't be.

The TL072 has nothing to do with the repeats, there shouldn't be any need to touch it.

EDIT: Also, please post pictures and voltages per the tech help rules.

[Battery Acid]

Good luck man, I just gave mine up

[rude]

When the path switch is off there is one repeat delay. When the path switch is on there is a filtered one repeat delay.

On the PT2399 chips I have the audio signal on pins 15,14, and 12 but not 16 and 13. Should there be an audio signal on pins 13 and 16? I tested this against a different ZPSDX that I think is working and did not have an audio signal on pins 13 and 16 on the PT2399 chips. What should it sound like on pins 13 and 16?

I didn't think that the one repeat issue would come from the filtering section. I checked the audio signal in the output pins of the TL074 and there is a signal but again, one repeat. I used the continuity test on my DMM to verify that the pins connecting the rotary switch do not have solder bridges.

Here are the voltages for the chips:

IC1 – TL072
1. 4.72 V
2. 4.72 V
3. 4.71 V
4. 0 V
5. 4.72 V
6. 4.72 V
7. 4.72 V
8. 9.43 V

IC2 – PT2399
1. 4.94 V
2. 2.48V
3. 0 V
4. 0 V
5.  2.92 V
6. 2.47 V
7. 0.7 V
8. 0.72 V
9. 2.48 V
10. 2.48 V
11.2.48 V
12. 2.47 V
13. 2.48 V
14. 2.49 V
15. 2.48 V
16. 2.48V

IC3 – PT2399
1. 4.94 V
2. 2.47 V
3. 0 V
4. 0 V
5. 2.92 V
6. 2.46 V
7. 0.68 V
8. 0.70 V
9. 2.47 V
10. 2.47 V
11. 2.47 V
12. 2.46 V
13. 2.47 V
14. 2.47 V
15. 2.47 V
16.2.47 V

IC4 – TL074
1. 4.66 V
2. 4.66 V
3. 4.23 V
4.  9.35 V
5. 4.65 V
6. 4.66 V
7. 4.66 V
8.  4.66 V
9. 4.7 V
10.  4.7 V
11. 0 V
12. 4.7 V
13. 4.67 V
14. 4.67 V

[rude]

Here are some pics...
http://www.flickr.com/photos/ryanisradd/sets/72157638367024426/

[jimilee]

Awww man, with all the ZPSDX issues lately, I'm sure I'll run into a few more when I start my build. That board has been sitting on my desk for way too long.

It is a more challenging build for sure, but you can get it.

[rude]

I used my audio probe to verify that the signal is going through the feedback pot and it is. I am stumped.

Does anybody have any ideas as to the source of the problem of one repeat in a delay? I am also having this problem with a Cave Dweller I built. I have successfully put together both the ZPSDX and the Cave Dweller in the past...but this one repeat issue is getting the best of me.

Help please. Before I pull out all of my hair.

[kothoma]

the signal is going through the feedback pot

You're saying you have signal at lug 2?
Check C48 and R41.

[midwayfair]

I am also having this problem with a Cave Dweller I built. I have successfully put together both the ZPSDX and the Cave Dweller in the past...but this one repeat issue is getting the best of me.

You're having exactly the same problem in two different delays?

If too much signal gets sent to ground by the feedback pot, you won't get any repeats. Did you happen to use the same potentiometers for both pedals?

[rude]

Same problem, two delays. Isn't that strange? I used different potentiometers for both pedals too.

So last night when I checked I thought there was an audio signal on lug 2 of the fdbk pot but I checked just now and there is no signal there. The signal is faint as it goes through R41, it is going into C48 but it is not passing through C48 into the fdbk pot. What should the signal sound like through R41?

I used a big 100V greenie for C48 . Do you think that type of cap may be causing a problem? I have used the same cap in other ZPSDXs that work...I'm not sure if I should just replace C48 with a different greenie or if I should get a different type of 220 nF capacitor.

[midwayfair]

Same problem, two delays. Isn't that strange? I used different potentiometers for both pedals too.

So last night when I checked I thought there was an audio signal on lug 2 of the fdbk pot but I checked just now and there is no signal there. The signal is faint as it goes through R41, it is going into C48 but it is not passing through C48 into the fdbk pot. What should the signal sound like through R41?

I used a big 100V greenie for C48 . Do you think that type of cap may be causing a problem? I have used the same cap in other ZPSDXs that work...I'm not sure if I should just replace C48 with a different greenie or if I should get a different type of 220 nF capacitor.

Composition of the caps won't matter.

This is actually good news that you've found a place where you're losing signal. It makes it more likely that the problem can be found.

R41 is a limit on the feedback. Let's examine the feedback path. Signal exits the tone shaping after the mode switch and is divided between the path (when off) and feedback pot. One repeat always goes to the mix pot.

The feedback pot is a voltage divider between the output at the mode switch and the input of the PT2399 stage. It's just like a volume pot; when you turn it down, resistance is 0K between the "output" back into the delay section and ground; when it's fully up, resistance to ground is at its maximum (the most signal is retained), and there is zero resistance between the delay output and R41. The slam switch jumpers the lugs, just like you'd dimed the feedback pot.

You've essentially traced the signal backwards in your description. The faint signal you're hearing on the R8/R41 junction is the dry signal. It's very faint because you've only got 5.1K resistance to ground there! And you don't hear anything on the other side of R41 because the signal is grounded. In fact, I bet if you switched to the continuity setting on your multimeter, you'd find that lug 2 of your feedback pot is grounded.

Unfortunately, I don't have a trace mask, BUT, reflow your solder on everything that touches the feedback pot. Lift D4/D5 maybe to make sure that one didn't blow. There's a via right next to the diodes -- make sure there's not a solder joint. It might be really, really small.

[rude]

Jon you are the best! I replaced C48 and I replaced D4 and D5....now I have a beautiful multitude of repeats pouring out of my ZPSDX. Thanks again. This one repeat issue has been ailing me for months.

Now to find the source of the one repeat issue in the Cave Dweller I am working on...